Method of reducing pressure drop during the passage of a fluid, and a hydraulic system reservoir for circulation of a fluid

ABSTRACT

In mobile hydraulic systems there is used a fine-mesh net structure (7) in a reservoir (6) for extracting air from the hydraulic fluid. When the system is started-up under cold-start conditions, a high pressure drop will prevail across the net structure, due to the viscosity of the fluid under such conditions. To overcome the problems associated herewith, a constricted passageway is provided in the net structure or adjacent thereto, such as to effect viscosity-dependent shunting of the fluid. This constricted passageway may have the form of a hole (11). A diffusor (13) may optionally be arranged in the reservoir, to ensure that the fluid will have laminor flow. A constricted passageway (11) of the aforesaid kind may also be provided in or adjacent to the diffusor (13).

FIELD OF INVENTION

The present invention relates to a method of reducing pressure drop inthe passage of a fluid across a fine-mesh net structure, for the purposeof extracting gas from the fluid, e.g. when starting-up mobile hydraulicsystems, preferably in the case of repeated passage of the fluid throughthe net structure.

The invention also relates to a reservoir intended for use influid-circulating hydraulic-systems and being equipped with an airseparator.

Effective separation of air from the working fluid of a hydraulic systemwill reduce the amount of input energy required by the system and willalso improve the precision at which the system operates. In order forair and other forms of gas to be separated effectively from hydraulicsystems, it is normally necessary to ensure that the rate of flowthrough the system reservoirs is so slow as to afford time for the airbubbles, or gas bubbles, to rise to the liquid surface of the reservoirsthereof, which means that large reservoirs are required. However, thespace available for the inclusion of reservoirs in such systems is oftenlimited, and consequently it is necessary to solve the problem of airseparation in some other manner; this problem is accentuated when thereservoir volume is reduced and the fluid through flow rate increased.

BACKGROUND PRIOR ART

For the purpose of extracting air effectively from fluid which passesthrough the reservoir of a hydraulic system at elevated rates of flow,it has been proposed to position a fine-mesh net structure between twomutually opposite corners of the reservoir, with the net structurearranged in an inclined position.

The results of trials carried out with the aid of such a net structurehave shown that, in addition to flow rate, the ability to separate aireffectively from hydraulic fluids is also contingent on the viscosity ofthe fluid concerned, the angle at which the net structure is inclinedand the fineness of the mesh.

Since in the case of closed systems the oil, or hydraulic fluid, willpass through the net structure several times, air separation willapproach a final value asymptotically. This final value will be higherwith increasing mesh fineness, since the air bubbles are divided inaccordance with size.

Consequently, in order to separate air effectively, it is necessary touse a net of vary fine mesh. However, the pressure drop across fine-meshnet structures is higher than in other cases, particularly whencold-starting mobile hydraulic systems, in which a small reservoir isdesirable for several reasons. In practice, the high viscous fluidtherethrough, a situation which is quite normal with the cold-start ofhydraulic systems, will result in problems of such gravity that othersolutions must be sought.

Those solutions proposed hitherto, however, have been both expensive andcomplicated and have not taken into account the fact that a fine-meshnet structure constitutes and effective means for separating gas from acontinuously operating hydraulic system, i.e. subsequent to solvingcold-start problems.

Solutions hitherto proposed which recommend the use of different typesof pressure limiters comprising moveable components can cause cavitationproblems and result in unsatisfactory gas separation.

OBJECT OF THE INVENTION

The object of the present invention is to avoid the aforesaid drawbacksand the invention, to this end, is based on the realization that sincethe fluid circulates in the hydraulic system, it is not necessary toextract all air at one and the same time, i.e. 100% extraction, but thatthe air can equally as well be extracted successively as the systemdeparts from the abnormal conditions which prevail in the case ofcold-starts.

One object of the invention in accordance herewith is to provide amethod of the aforesaid kind which will enable a fine-mesh net structureto be used in a reservoir of small volume while avoiding the problemsoccurring when cold-starting such systems.

Another object is to provide a method of the aforesaid kind which avoidsthe use of moveable parts and/or complicated construction elements tothe greatest possible extent.

SUMMARY OF THE INVENTION

The inventive method, which is effective in eliminating the aforesaidproblems, is characterised in its widest aspect, by passing a part ofthe fluid through a constricted passageway, provided with constrictiondefining means and located in or adjacent to the net structure, suchthat shunting of the fluid will be viscosity dependent.

The invention makes shunting of the fluid possible without the use ofmoveable parts, by utilizing the differences in characteristics ofdensity-dependent throttling and viscous throttling of the fluid flowand by coupling the same in parallel.

A theoretical explanation of the advantages afforded by the inventioncan be had with the aid of the following mathematical relationship. Thepressure drop across a net has a viscous characteristic and can beexpressed by the following equations:

    ΔP=K·Q·μ                        (1)

    μ=ν·ρ                                   (2)

in which

ΔP=pressure drop

K=a constant which is dependent on the area and geometry of the net

Q=the flow through the net

μ=the dynamic viscosity

ρ=the density

ν=the kinetic viscosity.

Density throttling can be achieved with the aid of a sharp-edged hole,which may be located in the actual net structure itself or adjacentthereto, e.g. in the edge region of the net structure.

The drop in pressure experienced across a constricted passageway, ornozzle means, in the form of a sharp-edged hole can be expressed by thefollowing equation: ##EQU1## in which ΔP=pressure drop

Q=flow through the constriction defining means

A=constriction defining means area

α=through flow index

Re=Reynolds number

ρ=density

It follows from this that in the case of density-dependent throttling##EQU2## whereas in the case of viscous throttling ##EQU3##

When the net structure and the constriction defining means according tothe invention are connected in parallel, the pressure drop across thenet will be equal. Consequently, the flow through the net and theconstricted passageway will vary in dependence on the kinematicviscosity. When starting up a cold hydraulic system in the open air,conditions under which the oil will have a high kinematic velocity, themajor part of the flow will pass initially through the constrictiondefining means and will be successively steered to pass through the netas the hydraulic fluid is warmed and its kinematic velocity subsequentlydecreases.

Consequently, the pressure drop will be lower during the startprocedure, since the major part of the fluid will flow through theconstriction defining means as opposed to all the fluid passing throughthe net. When the fluid is warm, after having been in work for someperiod of time, the major part of the fluid, will pass through the net.Since the fluid will pass repeatedly through the reservoirs, the finalasymptomic air-separation value will be approximately equal to the valueobtained when passing all of the fluid through the net, without firstshunting the fluid through the constriction defining means.

It will be understood from the aforegoing that said part of the fluidcan be conducted through a sharp-edged hole which is located in the netstructure and which forms the constricted-passageway therein.Alternatively, the fluid may be passed to a constricted passagewaylocated at an edge region of the net structure.

When the hydraulic system includes an number of small reservoirs, as isdesired in accordance with the invention, with correspondingly largethrough-flows of fluid and commensurately high flow rates, it isnormally necessary to install a diffusor in the fluid flow path, saiddiffusor suitably having the form of a perforated plate effective inimparting laminar flow to the fluid prior to its entry into the netpassage.

The area of the constricted passageway used in the system willpreferably be much larger than the total area of the perforations in thediffusor, and in order to achieve maximum possible efficiency in theflow sequence, the constricted passageway should be positioned so thatthe flow path between the diffusor and the constricted passageway is theshortest possible.

In this respect it lies within the scope of the invention to place theconstricted passageway in the diffusor or adjacent thereto, which willtherewith satisfy the aforesaid requirement that the constrictedpassageway is located "in connection with the net structure".

Thus it also lies within the scope of the invention to locate theconstricted passageway in an edge region of the diffusor and/or aconstricted passageway in the net structure or in an edge regionthereof.

The diffusor and net structure may optionally be positioned immediatelyadjacent one another, such that a common edge region of both of saidcomponents will form means for defining the constricted passagewayrequired by the invention.

Thus, it is essential in this respect that that part of the fluid whichis intended to pass through the constricted passageway under cold-startconditions will be led in a direction towards said passageway, and thatthe diffusor is used to this end. The fact that turbulence may occur inthe flow at precisely this position of the system will have no influenceon the desired extraction of air from the system, since this extractionis achieved nevertheless during the continued extraction process.

The invention also relates to hydraulic system reservoirs for hydraulicsystems in which a fluid is circulated, the characterised features ofthe reservoirs being set forth in the following apparatus claims.

The invention will now be described with reference to a number ofexemplifying embodiments thereof and with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a mobile hydraulic system together withits essential components.

FIG. 2 is a perspective view of one embodiment of a reservoir for thehydraulic system illustrated in FIG. 1, the Figure also showing thepattern of fluid flow occurring when the system is started-up under coldconditions, i.e. when the fluid is cold.

FIG. 3 corresponds to FIG. 2 and illustrates the flow pattern occurringwhen the fluid is warm.

FIG. 4 illustrates an alternative reservoir arrangement which includes adiffusor.

FIGS. 5-7 are perspective views of various alternative constrictedpassageway arrangements in an inventive reservoir.

The hydraulic system illustrated in FIG. 1 in intended to serve mobilehydraulics in open-air locations, where cold starts may proveproblematic.The system comprises a pump 2 which is driven by a motor 1and which is operative in delivering fluid, oil, under pressure toapparatus 4, not shown in detail, through a pipe 3.

The return pipe or line of the hydraulic system is reference 5 and isconnected to a reservoir 6, in which there is arranged a fine-mesh netstructure 7, intended for extracting air from the fluid, and a diffusor8.The net structure 7 and the diffusor 8 are positioned so as to liebeneath the level 9a of the oil in the reservoir.

FIG. 2 illustrates, in greater detail, a first embodiment of thereservoir 6 which lacks the provision of a diffusor. The reservoir hasan inlet 5a and an outlet 10a, between which there is arranged afine-mesh air-separating net structure 7.

When the system is started-up under cold conditions, when the oil 9 hasa high viscosity, the pressure drop across the net will be excessivelyhigh.In order to avoid the disadvantageous consequencies of such a highpressuredrop, a sharp-edged hole 11 is provided in the net structure 7.The hole functions to define a constricted passageway means andengenders viscosity-dependent shunting of the fluid. When the apparatus4 is started-up, the major part of the fluid will pass through the hole11, therewith lowering the pressure drop across the net structure 7.

FIG. 3 illustrates the course taken by said fluid flow after the systemhasbeen in operation for some time and the fluid, oil, has become warnand itsviscosity has decreased. The fluid flow is now more uniformlydivided and the fluid will flow through both the hole 11 and the net 7.

FIG. 4 illustrates an embodiment in which a diffusor 13 is positionedabovethe inlet 5a, to ensure laminar flow of the fluid in the reservoir6.

The diffusor 13 has perforations 14 and includes a downwardly slopingsurface 13a located immediately above the hole 11 in the net structure7. As with the embodiment aforedescribed, the major part of the fluid inthiscase will also flow through the hole 11 in the net 7, when thesystem is started-up under cold conditions.

In order to facilitate such flow, the diffusor 13 may also be providedwitha corresponding hole 11 which serves as a flow constrictionpassageway. In this case, the two holes 11 are preferably positioned sothat the flow path therebetween will be the shortest possible.

FIG. 5 illustrates an embodiment in which a constricted passageway, inthe form of a sharp-edged hole or aperture 11' is instead formed in anedge region of the net structure 7 and the diffusor 13 respectively. Inthe case of the FIG. 5 embodiment, the two constricted passageways 11are located at respective lower edges of the diffusor 13 and the netstructure7, and are consequently spaced only a short distance apart.

FIG. 6 illustrates an embodiment in which the net structure and thediffusor have mutually contacting, sharp lower edged which form aconstricted passageway 11', said means extending across the whole widthofthe reservoir.

FIG. 7 illustrates an embodiment which corresponds essentially to theembodiment illustrated in FIG. 6, but in which both the diffusor 13 andthe net structure 7 have inwardly cut recessed portions 13b and 7b,these inwardly cut recesses also serving to define further constrictedpassageways for viscosity-dependent shunting of the fluid undercold-startconditions.

It will be seen from the aforegoing that the fundamental inventiveconcept presented in the introduction can be manifested in manydifferent forms. Although the effectiveness of the described embodimentsmay be expected tovary, all the embodiments will nevertheless fulfillthe purpose intended.

Other embodiments are conceivable within the scope of the basicinventive concept. For instance, several constriction defining means maybe providedin or adjacent to the net structure, instead of one singleconstriction defining means, or constricted passageway. Furthermore, thediffusor can be given a configuration different to that described andillustrated.

I claim:
 1. A method of reducing pressure drop across a fine-mesh netstructure during the passage of a fluid through said net structure, forthe purpose of extracting air or gas from the fluid, characterised byconducting part of the fluid flow through a constricted passagewaylocated in or adjacent to the net structure, so as to effectviscosity-dependent shunting of the fluid.
 2. A method according toclaim 1, characterised by conducting said part of said fluid flowthrough a sharp-edged hole in the net structure.
 3. A method accordingto claim or claim 2, characterised by conducting said part of said fluidflow through a constricted passageway located in an edge region of thenet structure.
 4. A method according to claim 1, characterised by alsocausing the fluid to flow through a diffusor having the form of aperforated plate and being effective in imparting laminar flow to thefluid upstream of the net passageway.
 5. A method according to claim 4,in which the constriction has a larger area than the area of theperforations, characterised by positioning the constricted passageway ina manner to achieve the shortest possible flow distance between saidpassageway and said diffusor.
 6. A method according to claim 1, appliedto starting-up mobile hydraulic systems, wherein the fluid is repeatedlypassed through the net structure.
 7. A reservoir (6) for use influid-circulating hydraulic systems and provided with air separatormeans, said means including a fine-mesh net structure (7) which islocated in the path of fluid flow and which slopes in relation to thesurface of the fluid in said reservoir, said reservoir having a fluidinlet (5a) and a fluid outlet (10a), characterised by a constricted flowpassageway (11, 11') located in or adjacent to the net structure, forviscosity-dependent shunting of the fluid (9).
 8. A reservoir accordingto claim 7, characterised in that the passageway includes a sharp-edgedaperture (11, 11') provided in the net structure or in an edge regionthereof.
 9. A reservoir according to claim 7 or claim 8, characterisedin that the reservoir has arranged therein a diffusor (13) whichincludes a perforated plate which is effective in imparting laminar flowto the fluid upstream of the net passageway.
 10. A reservoir accordingto claim 9, characterised in that the constricted passageway and/or afurther constricted passageway is provided in the diffusor or in an edgeregion thereof.
 11. A reservoir according to claim 10 with a passagewayin or adjacent to the diffusor (13) and a passageway in or adjacent tothe net structure (7), characterised in that the passageways (11, 11')are located such as to present a short flow path therebetween.
 12. Areservoir according to claim 9, characterised in that at least the majorpart of the net structure (7) and the diffusor (13) are located beneaththe surface of the fluid in the reservoir.